physiology

• SNS has relatively long postganglionic fibres
• PNS has relatively short postganglionic fibres
• SNS originates from thoraco-lumbar region
• PNS originates from cranio-sacral region
• Postganglionic transmitter in SNS is noradrenaline
• Postganglionic neurotransmitter in PNS is acetylcholine
• Not every organ or tissue that has a sympathetic innervation also has a parasympathetic innervation.
• Skin potentials recorded over the palm and sole of the foot assess sympathetic activity in peripheral nerves
• Stimulation of sweating increases skin potential due to sympathetic activation of eccrine sweat glands on palmar surfaces
• Stimulation of the median nerve can be on either side of the body to the measured potential because it is stimulating afferent fibres for the sweating response reflex, which are completely independent to the efferent fibres.
• PNS preganglionic neurotransmitter is always acetylcholine
• heart rate variability is determined by both vagal and sympathetic innervation of the heart
• Blood accumulates in the lower limbs on standing, leading to a decreased venous return, resulting in decreased stroke volume and falling systolic blood pressure. The decreased central venous pressure is detected by atrial stretch receptors and the decreased systolic pressure is detected by arterial baroreceptors. This activates the baroreceptor reflex which acts to maintain the BP relatively constantly in the short term.
• Sweat production can be assessed by two methods:

  1. Measuring the sympathetic skin response (SSR): the change in the potential difference across a hand or foot following peripheral nerve stimulation. 
  2. Measuring an increase in skin conductance: the electrical resistance between two electrodes when a very weak current passes between them
• In a person with autonomic dysfunction, there is a diminished change in skin potential difference
• the rr interval is used to assess the integrity of the autonomic nerves supplying the heart (measure heart rate variability). Graphed as a poincare plot
• In a poincare plot, the greater the scatter, the greater the beat-beat variation in heart rate
• A person with loss of peripheral autonomic function would have less HRV compared to a normal person
• The valsalva manoeuvre reduces heart rate and increases blood pressure due to the increase in thoracic pressure.
• sympathetic stimulation increases heart rate and contractility
• parasympathetic stimulation decreases heart rate and contractility
• What is the first phase of the normal physiological response to a Valsalva maneuver?

  Heart rate decreases slightly.
• What effect does increased intrathoracic pressure have during the Valsalva maneuver?

  It drives blood from the lungs to the left atrium.
• What is the result of blood being driven to the left atrium during the Valsalva maneuver?

  It results in an increased stroke volume.
• How does venous return change during the Valsalva maneuver?

  Venous return is reduced, leading to a fall in stroke volume and systolic blood pressure.
• What role do baroreceptors play during the Valsalva maneuver?

  They detect the decrease in blood pressure and compensate with peripheral arteriolar vasoconstriction.
• What happens to heart rate as a result of baroreceptor activity during the Valsalva maneuver?

  Heart rate increases.
• What occurs after the pressure is released during the Valsalva maneuver?

  Blood returns to the thoracic cavity, causing an initial decrease in stroke volume.
• What happens to blood pressure and heart rate after the Valsalva maneuver is completed?

  They return to normal.
• What is the effect on cardiac output after the Valsalva maneuver?

  Cardiac output returns to normal.
• How many phases are there in the normal physiological response to a Valsalva maneuver?

  Four phases.
• What physiological changes occur in the first phase of the Valsalva maneuver?

  Heart rate decreases slightly, and blood is driven from the lungs to the left atrium, increasing stroke volume.
• How do the changes in stroke volume relate to blood pressure during the Valsalva maneuver?

  As stroke volume decreases, systolic blood pressure also falls progressively.
• In a person with autonomic dysfunction, during the valsalva manoeuvre, blood pressure falls and remains low until intrathoracic pressure is restored. Changes in heart rate would be reduced.
• 1. decrease in venous return from lower limbs
  2. decrease in stroke volume
  3. fall in systolic blood pressure
  4. activation of baroreceptor reflex
  5. increase in sympathetic activity
  6. increase in heart rate and force of contraction
• Changes in heart rate and blood pressure peak about 15 seconds after standing from supine, but will usually return to normal after 40–60 seconds.
• orthostatic hypotension is when people have decreases of 20mmHg systolic or 10mmHg diastolic within 3 minutes of standing
• Two optic nerves carry light information from the eyes to the midbrain.

  Two oculomotor nerves (including sympathetic and parasympathetic fibers) carry information from the brain to each eye.
• Sympathetic fibres innervate the radial muscles of the iris - pupil dilation - and the muscles of the eyelids to hold the upper eyelid open.
• Parasympathetic fibres innervate the circular muscles of the iris - contraction of pupil